Structured metal parts made of titanium or titanium alloys are conventionally made by casting, forging or machining from a billet. These techniques have a disadvantage of high material waste of the expensive titanium metal and large lead times in the fabrication of the metal part.
Fully dense physical objects can be made by a manufacturing technology known as rapid prototyping, rapid manufacturing, layered manufacturing, solid freeform fabrication, additive fabrication, additive manufacturing or 3D printing. This technique employs computer aided design software (CAD) to first construct a virtual model of the object which is to be made, and then transform the virtual model into thin parallel slices or layers, usually horizontally oriented. The physical object can then be made by laying down successive layers of raw material in the form of liquid paste, powder or other layerable, spreadable or fluid form, such as melted metal, e.g., from a melted welding wire, or preformed as sheet material resembling the shape of the virtual layers until the entire object is formed. The layers are fused together to form a solid dense object.
Solid freeform fabrication is a flexible technique allowing creation of objects of almost any shape at relatively fast production rates, typically varying from some hours to several days for each object. The technique is thus suited for formation of prototypes and small production quantity, and can be scaled-up for large volume production.
The technique of layered manufacturing can be expanded to include deposition of pieces of the construction material, that is, each structural layer of the virtual model of the object is divided into a set of pieces which when laid side by side form the layer. This allows forming metallic objects by welding a wire onto a substrate in successive stripes forming each layer according to the virtual layered model of the object, and repeating the process for each layer until the entire physical object is formed. The accuracy of the welding technique is usually too coarse to allow directly forming the object with acceptable dimensions. The formed object will thus usually be considered a green object or pre-form which needs to be machined to acceptable dimensional accuracy.
Electron beam freeform manufacturing is known in the art (e.g., see Taminger and Hafley (“Characterization of 2219 Aluminum Produced by Electron Beam Freeform Fabrication,” Presented at the 13th Solid Freeform Fabrication Symposium, Aug. 5-7, 2002, Austin, Tex.; In proceedings, University of Texas at Austin (2002); Taminger and Hafley (“Electron Beam Freeform Fabrication: A Rapid Metal Deposition Process,” Proceedings of the 3rd Annual Automotive Composites Conference, Sep. 9-10, 2003, Troy, Mich., Society of Plastics Engineers (2003); and Taminger and Hafley (“Electron Beam Freeform Fabrication for Cost Effective Near-Net Shape Manufacturing”, NATO/RTOAVT-139 Specialists' Meeting on Cost Effective Manufacture via Net Shape Processing (Amsterdam, the Netherlands, 2006) (NATO). pp 9-25)). Taminger and Hafley (2006) describes a method and device for manufacturing structural metal parts directly from computer aided design data combined with electron beam freeform fabrication (EBF). The structural part is built by welding on successive layers of a metallic welding wire which is welded by the heat energy provided by the electron beam. The EBF process involves melting a metal wire into a molten pool made and sustained by a focused electron beam in a high vacuum environment. The positioning of the electron beam and welding wire is obtained by having the electron beam gun and the actuator supporting the substrate movably hinged along one or more axis (X, Y, Z, and rotation) and regulate the position of the electron beam gun and the support substrate by a four axis motion control system. The process is claimed to be nearly 100% efficient in material use and 95% effective in power consumption. The method can be employed both for bulk metal deposition and finer detailed depositions, and the method is claimed to obtain significant effect on lead time reduction and lower material and machining costs as compared to the conventional approach of machining the metal parts. The electron beam technology has a disadvantage of being dependent upon a high vacuum of 10−1 Pa or less in the deposition chamber.
It is known (e.g., see Adams, U.S. Pat. Pub. No. 2010/0193480) to use a TIG-welding torch to build objects by solid freeform fabrication (SFFF), where successive layers of metallic feedstock material with low ductility are deposited onto a substrate. A plasma arc is created by energizing a flowing gas using an electrode, the electrode having a variable magnitude electric current supplied thereto. The plasma stream is directed towards a predetermined targeted region to preheat the predetermined targeted region of the workpiece prior to deposition. The electric current is adjusted and the feedstock material is fed into the plasma stream to deposit molten feedstock in the predetermined targeted region. The current is adjusted and the molten feedstock is slowly cooled at an elevated temperature, typically above the brittle-to-ductile transition temperature of the feedstock material, in a cooling phase to minimize the occurrence of material stresses.
Withers et al. (U.S. Pat. Pub. No. 2006/185473) also describes using a TIG torch in place of the expensive laser traditionally used in a solid freeform fabrication (SFFF) process with relatively low cost titanium feed material by combining the titanium feed and alloying components in a way that considerably reduces the cost of the raw materials. Withers et al. also describes using titanium sponge material mixed with alloying elements formed into a wire where it can be used in an SFFF process in combination with a plasma welding torch or other high power energy beam to produce near net shaped titanium components.
Abbott et al. (WO 2006/133034, 2006) discloses a direct metal deposition process using a laser/arc hybrid process to manufacture complex three-dimensional shapes comprising the steps of providing a substrate and depositing a first molten metal layer on the substrate from a metal feedstock using laser radiation and an electric arc is disclosed. The electric arc can be provided by gas metal arc welding using the metal feedstock as an electrode. Abbott et al. teaches that the use of laser radiation in combination with gas metal arc welding stabilizes the arc and purportedly provides higher processing rates. Abbott et al. utilizes a metal wire guided by and exiting out of a wire guide. The metal of the metal wire is melted at the end and the molten metal is deposited by positioning the end over the deposition point. The required heat for melting the metal wire is supplied by an electric arc expanding between the tip of the electrode and the workpiece/deposition substrate, and by a laser irradiating the deposition area. Welding by melting a metal wire heated by an electric arc is known as gas metal arc welding (GMAW), of which in the case of using non-reactive gases to make the arc is also denoted as metal inert gas welding (MIG-welding).
A problem to be addressed is the speed of deposition of material on the base material to form the workpiece. One could increase the temperature of the metal wire to preheat the metal wire before it interacts with the arc of the arc torch. This could be achieved by increasing the flow rate of electric charge through the electrode (amperes of current) or modulating the cross-section of the metal wire to increase the resistive heating of the electrode. Titanium metal or titanium alloys heated above 400° C. may be subject to oxidation upon contact with oxygen. It is thus necessary to protect the weld and heated object which is being formed by layered freeform manufacture against oxygen in the ambient atmosphere.
Use of high current, however, can produce a number of problems. If the change in current is not controlled, a rapid overheating of the metal wire can occur, resulting in burn back of the metal wire to the contact tip. The burn back can result in the fusion of the metal wire with the contact tip, which would necessitate replacement of the contact tip. Use of high current also can cause the contact tip itself to heat up and can result in overheating of the contact tip. One of the results of overheating of the contact tip can be tip elongation. Depending on the configuration of the contact tip, such elongation could result in the contact tip being repositioned closer to the metal wire wire, which can increase the friction between the contact tip and the metal wire, would could damage or scratch the metal wire wire. Modulation in contact tip geometry caused by overheating also can result in uneven contact tip wear because of thermal induced elongation or unevenness. This can lead to the formation of electric arc formation within the contact tip. Overheating of the contact tip also can result in the formation of micropores in the contact tip, which can cause the welding apparatus to operate erratically.
These problems that can result from use of high currents at the contact tip can result in the need for frequent replacement of contact tips and, in worst case scenarios, cleaning of the welding apparatus. Replacement of the contact tip and/or cleaning of the apparatus requires shutting down the apparatus, halting production. This is expensive and negatively impacts productivity.
There also exists a need in this art for an economical method of performing direct metal deposition at an increased rate of metal deposition. There further exists a need in this art for an apparatus that allows increased throughput and yield of direct metal deposition formed products without the risk of frequent contact tip replacement due to overheating.